(1) Field of the invention
The present invention concerns a rechargeable lithium anode for polymer electrolyte batteries. More particularly, the invention is concerned with an electrochemical generator, for example a generator, which operates with an anode of lithium or other alkali metals, pure or alloyed, and a polymer electrolyte and whose characteristics are such that it is possible to substantially increase the number of cycles obtained during the normal life of the battery.
(b) Description of Prior art
The rechargeability of lithium in the presence of organic electrolytes generally leads to an important morphological evolution of the anode of an alkali metal, such as lithium resulting in a loss of utilization of the anode and/or the appearance of dendrites during recharge. This phenomenon is known to be very general with all generators in which metallic lithium is used. Reference is made to the following: Industrial Chemistry Library, Volume 5, LITHIUM BATTERIES, New Materials, Developments and Perspectives, Chapter 1, authored by J. R. Dahn et al. edited by G. Pistoia, Elsevier (1994). The generally acceptable explanation for this phenomenon is based on the following:
1--lithium is thermodynamically unstable in the presence of organic electrolytes (solvent+lithium salt) and produces an oxidation layer (which is more or less passivating and, under certain conditions, remains a conductor for lithium ions); in the latter case the passivating layer is designated SEI: Solid Electrolyte Interface;
2--during consecutive cycles of discharge and recharge, lithium is dissolved and is re-deposited unevenly and at the end, becomes electrically insulated and/or is chemically consumed, by reaction with the organic electrolyte (solvent and salt);
3--the result is a loss of activity for the anode and the formation of a porous and irregular anode consisting of dispersed lithium, which is more or less passivated.
In general, this phenomenon is compensated by increasing the capacity of the lithium anode with respect to the cathode so as to obtain a significant number of charges and discharges during cycling. The term capacity of the anode means the "coulomb" capacity, i.e., the quantity of electricity present in the anode, knowing that one mole of lithium contains the equivalent of 96,500 coulombs. Normally, this value is defined in cm.sup.2, for example, 10 coulombs/cm.sup.2. The capacity may also be defined in milliampere-h/cm.sup.2 and the conversion is carried out as follows: 1 mAh/cm.sup.2 =3.6 C/cm.sup.2.
The concept generally used to define the general behavior associated with the difficulty of redepositing lithium with a yield of 100% is called F.O.M. (Figure of Merit) Second International Symposium on Polymer Electrolytes, Siena, Italy, Jun. 14-16, 1989, Belanger et al.
For these reasons, lithium anodes whose capacity is 3 to 8 times the capacity of the corresponding cathode, are generally used in secondary generators. This procedure enables to obtain a significant number of discharge/charge cycles, however, it substantially reduces the density of energy of the generator because of the penalty associated with an excess volume of lithium. Moreover, an excess of lithium substantially increases the cost of the generator. On the other hand, the risks associated with handling rechargeable Li batteries are higher when the Li excess number is higher, more so if cycling is accompanied with an important morphological evaluation of lithium which makes it more reactive.
The term "morphology" applied to lithium anodes is a description of the roughness of the surface developed during cycling. This surface roughness in some cases extends within the body of the anode when the latter becomes porous during cycling. Therefore, when the morphology of a surface is developed this also means that it becomes coarser. Many apparatuses are known for scanning the surface of an electrode to determine its roughness to .+-.1 micron.
It has been observed that a development of morphology of lithium may also take place in the presence of a polymer electrolyte although at a lesser degree. This observation has confirmed the evolution of the morphology of an anode of lithium, when it is cycled under repetitive conditions. Applicants have illustrated this phenomenon in a previous work (Siena, Belanger et al., cf. supra). Relatively high F.O.M. values noted during tests made wherein F.O.M. &gt;100, have, however, led to using an excess of lithium close to or higher than three times the capacity of the cathode.
As a consequence, relatively thick lithium sheets are generally used in metallic lithium rechargeable generators for reasons of commercial availability and ease of handling. It is also possible to use a rigid current collector which is applied against the film of lithium in order to facilitate its handling and to provide for an electrical contact.
In order to optimize the designs and performances of polymer electrolyte generators, and to provide a generator capable of storing enough energy for an electrical vehicle, the Applicant has designed a process of manufacturing lithium films which are increasingly thin. Reference is made to U.S. Pat. Nos. 4,517,265; 4,652,506; 4,794,060; and 4,897,917, as well as U.S. application Ser. Nos. 08/273,756 and 08/273,759. At the same time, Applicant has removed the rigid current collector so as to optimize the production cost and the energy density of the generators.
By way of example, Applicant has produced devices in the laboratory of a few mWh as well as other devices of more than 10 Wh utilizing lithium films without collectors in which the thickness varies between 20 and 40.mu. (J. Power Sources, 54 (1995) 163).
In all cases, good performances during cycling are obtained by providing an anode whose capacity is three times higher than that of the corresponding cathodes, and sometimes even more. Under similar conditions, when there are used films of lithium which are thinner and whose capacity is lower (capacity three times higher than that of the cathode), a rapid reduction of the number of cycles have been observed. After dismantling these generators, an important morphological evolution of the thin film of lithium has been noted, which is visible by a simple observation to the eye or by observation with a scanning electronic microscope. In the case of these very thin sheets, the morphological evaluation is visible throughout the entire sheet.
The morphological evolution of lithium is particularly fast under the following conditions:
1--when the excess of lithium is small;
2--when the film of lithium is thin &lt;30.mu.p, and 3--when lithium is free, i.e. non-supported by a rigid current collector.
These observations corroborate the tendency noted in the prior art and constitute a major limitation with respect to the optimization of the performances of generators having a metallic lithium anode, i.e., when it is intended to reduce the excess of the capacity of lithium with respect to that of the cathode, to remove the presence of a rigid collector, which is often thick and costly, or even when it is intended to reduce the thickness of the group of films constituting the generator: anode/polymer electrolyte/cathode/collector to optimize the power and the cyclability.
It is an object of the present invention to provide a rechargeable polymer electrolyte generator operating with an alkali metal anode, such as metallic lithium, and which is capable of undergoing repeated deep cycles of charge and discharge.
It is another object of the present invention to provide a generator in which the alkali metal anode has a low excess lithium capacity enabling it to preserve the reversible reaction of the anode and the electrical collection of the latter by maintaining the initial morphology without forming porous alkali metal or particles of electrically insulated metal. This characteristic of the invention is made possible by the absence of a substantial consumption of lithium by the polymer electrolyte during cycling under the conditions of the invention.
It is another object of the present invention to provide for the utilization of very thin lithium films, such as &lt;50.mu., while enabling to reduce and/or eliminate the excess of metal installed with respect to the cathode, and to maintain good cycling characteristics.
It is another object of the present invention to reduce and/or eliminate the excess of capacity of the lithium anode required with generators according to the invention as compared to the prior art, in terms of security, electrochemical performance, and cost associated with this excess of capacity of the anode.